MDL-37564 linked dependant plugin component text to plugins DB
MDL-41146 Backup: backup_includes_files can leak memory
MDL-38190 (1) Backup/restore: Progress tracking API
MDL-38190 (2) Backup/restore: Progress tracking for tasks and steps
MDL-38190 (3) Backup/restore: Progress display in main user interface
MDL-41304: Hide and handle LDAP error when user not in context being checked
MDL-33472 google plugins: move helper functions to upgradelib.php

upgrade.php is reserved for the upgrade steps only. See also
MDL-34103
MDL-41418 Administration: Page context should be set before setting page title
MDL-41369 Set proper context and navigation path for user messages
MDL-41317 Set correct context to send bulk message page
MDL-41256 course: improved debugging on course creation
MDL-41428 repository_coursefiles: Missing white space in name
MDL-41440 event: fixed incorrect treatment of get_string as a class
MDL-29954 repository: remove dead code

Also remove repository_attach_id() which was never used and not useful.
MDL-40214 mod_assign: Fixed issue with custom scale being lost during restore.
MDL-41348 question code depends on filelib.

I think this used to work because mod/forum/lib.php used to be included
everywhere, and in turn included lib/formslib.php. We should declare the
dependencey explicitly.
MDL-41403 Backup: reduce memory usage when searching for courses / categories

Also makes a few existing comments in the changed code area adhere to coding standards.
MDL-41485 calendar: Replace value of action buttons on managesubscription page with int
MDL-32690: Restore 1.9 backup into 2.X fails on missing assignment type

Solution is as follows:
* Allow unsupported subplugins to convert from Moodle1 to Moodle2
* On Moodle2 restore hide unsupported subplugins
* Graceful error message in assignment of unsupported subplugins
MDL-41456 Events: Fixed existing events to respect base event access specifier and return data type